Essentially, the heart is a pump which pumps blood throughout the body. It consists of four chambers--two atria and two ventricles. In order for the heart to efficiently perform its function as a pump, the atrial muscles and ventricular muscles should contract in a proper sequence and in a timed relationship.
In a given cardiac cycle (corresponding to one "beat" of the heart), the two atria contract, forcing the blood therein into the ventricles. A short time later, the two ventricles contract, forcing the blood therein to the lungs (from the right ventricle) or through the body (from the left ventricle). Meanwhile, blood from the body fills the right atrium and blood from the lungs fills the left atrium, waiting for the next cycle to begin. A typical healthy adult heart may beat at a rate of 60-80 beats per minute (bpm) while at rest, and may increase its rate to 140-180 bpm when the adult is engaging in strenuous physical exercise, or undergoing other physiologic stress.
The healthy heart controls its rhythm from its sino-atrial (SA) node, located in the upper portion of the right atrium. The SA node generates an electrical impulse at a rate commonly referred to as the "sinus" or "intrinsic" rate. This impulse is delivered to the atrial tissue when the atria are to contract and, after a suitable delay (on the order of 40-80 milliseconds), propagates to the ventricular tissue when the ventricles are to contract.
The detectable electrical signal which causes the atria to contract, is referred to as a P-wave, while the detectable electrical signal which causes the ventricles to contract is referred to as a QRS complex often abbreviates as an R-wave. The R-wave is much larger than the P-wave, principally because the ventricular muscle tissue is much more massive than the atrial muscle tissue. The atrial muscle tissue need only produce a contraction sufficient to move the blood a very short distance--from the respective atrium to its corresponding ventricle. The ventricular muscle tissue, on the other hand, must produce a contraction sufficient to push the blood over a long distance (e.g., through the complete circulatory system of the entire body).
The electrical signal or wave most commonly detectable within a cardiac cycle, is the T-wave (which represents the depolarization of the ventricular muscle tissue).
It is the function of a pacemaker to provide electrical stimulation pulses to the appropriate chamber(s) of the heart (atria or ventricles) in the event the heart is unable to beat on its own (i.e., in the event either the SA node fails to generate its own natural stimulation pulses at an appropriate sinus rate, or in the event such natural stimulation pulses do not effectively propagate to the appropriate cardiac tissue). Most modern pacemakers accomplish this function by operating in a "demand" mode where stimulation pulses from the pacemaker are provided to the heart only when it is not beating on its own, as sensed by monitoring the appropriate chamber of the heart for the occurrence of a P-wave or an R-wave. If a P-wave or an R-wave is not sensed within a prescribed period of time (referred to as the "escape interval"), then a stimulation pulse is generated at the conclusion of this prescribed period of time and delivered to the appropriate heart chamber via a pacemaker lead.
Modern programmable pacemakers are generally of two types: (1) single chamber pacemakers, and (2) dual-chamber pacemakers. In a single chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single chamber of the heart (e.g., either the right ventricle or the right atrium). In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart (e.g., both the right atrium and the right ventricle). The left atrium and left ventricle can also be paced, provided that suitable electrical contacts are made therewith.
In general, both single and dual-chamber pacemakers are classified by type according to a three letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e., the chamber where a stimulation pulse is delivered)--with a "V" indicating the ventricle, an "A" indicating the atrium, and a "D" indicating dual or both the atrium and ventricle. The second letter of the code identifies the chamber where cardiac activity is sensed, using the same letters to identify the atrium or ventricle or both, and where an "O" indicates that no sensing takes place.
The third letter of the code identifies the action or response that is taken by the pacemaker. In general, two types of action or responses are recognized: (1) an Inhibiting ("I") response, where a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger ("T") response, where a stimulation pulse is delivered to the designated chamber of the heart a prescribed period after a sensed event. Both of the abovedescribed responses may be combined resulting in a Dual ("D") response, where both the Inhibiting mode and Trigger mode are evoked, inhibiting in one chamber of the heart and triggering in the other.
A fourth letter, "R", is sometimes added to the code to signify that the particular mode identified by the three letter code is rate-responsive, where the pacing rate may be adjusted automatically by the pacemaker based on one or more physiological factors, such as blood oxygen level or the patient's activity level.
Thus, for example, a DVI pacemaker is a pacemaker that paces in both chambers of the heart, but only senses in the ventricle, and that operates by inhibiting stimulation pulses when prior ventricular activity is sensed. Because it paces in two chambers, it is considered a dual-chamber pacemaker. A VVI pacemaker, on the other hand, is a pacemaker that paces only in the ventricle. Because only one chamber is involved, it is classified as a single chamber pacemaker. Most dual-chamber pacemakers can also be programmed to operate in a single chamber mode.
Much has been written and described about the various types of pacemakers and the advantages and disadvantages of each. For example, commonly assigned U.S. Pat. No. 4,712,555 of Thornander et al. presents background information about pacemakers and the manner in which they interface with a patient's heart. This patent is hereby incorporated by reference in its entirety.
One of the most versatile programmable pacemakers available today is the DDDR pacemaker. This pacemaker represents a fully automatic pacemaker which is capable of sensing and pacing in both the atrium and the ventricle, and is also capable of adjusting the pacing rate based on one or more factors independent of the intrinsic electrical depolarization signals generated by the heart, but indicative of a need for a faster heart rate--such as the patient's activity level. When functioning properly, the DDDR pacemaker can limit certain drawbacks associated with the use of pacemakers. For example, the DDDR pacemaker can maintain AV synchrony while providing bradycardia support at progressively higher rates based upon physiologic requirements.
In general, DDD pacing has four functional states: (1) P-wave sensing, ventricular pacing (PV); (2) atrial pacing, ventricular pacing (AV); (3) P-wave sensing, R-wave sensing (PR); and (4) atrial pacing, R-wave sensing (AR). Advantageously, for the patient with complete or partial heart block, the PV state of the DDD pacemaker tracks the atrial rate which is set by the heart's SA node, and then paces in the ventricle at a rate that follows this atrial rate. Because the rate set by the SA node represents the rate at which the heart should beat in order to meet the physiologic demands of the body (at least for a heart having a properly functioning SA node) the rate maintained in the ventricle by such a pacemaker is truly physiologic.
Those skilled in the art have long recognized the advantages of using an atrial rate-based (i.e. P-wave tracking) pacemaker. For example, U.S. Pat. No. 4,624,260 to Baker, Jr. et al. discloses a microprocessor-controlled dual-chamber pacemaker having conditional atrial tracking capability. Similarly, U.S. Pat. No. 4,485,818 of Leckrone et al. discloses a microprocessor-based pacemaker which may be programmed to operate in one of a plurality of possible operating modes, including an atrial rate tracking mode.
Unfortunately, in some instances, a given patient may develop fast atrial rhythms which result from a pathologic arrhythmia such as a pathological tachycardia, fibrillation, or flutter. In these cases, a DDD(R) pacemaker may pace the ventricles in response to the sensed atrial arrhythmia up to a programmed maximum tracking rate (MTR). The MTR defines the upper limit for the ventricular rate when the pacemaker is tracking the intrinsic atrial rate (IAR). As a result, the MTR sets the limit above which the ventricles cannot be paced, regardless of the intrinsic atrial rate. Thus, the purpose of the MTR is to prevent rapid ventricular stimulation, which could occur if the intrinsic atrial rate becomes very high and the pacemaker attempts to track intrinsic atrial activity with 1:1 AV synchrony.
Pacemakers also incorporate a programmable parameter known as an atrial refractory period, which is initiated by either a paced or sensed cardiac event. The atrial refractory period, also called a total atrial refractory period (TARP), is made up of two segments. The first segment, known as the AV Delay (AVD), is initiated by a paced or sensed atrial event. The second segment, known as the post ventricular atrial refractory period (PVARP), is initiated by a paced or sensed ventricular event. The TARP (measured in milliseconds (ms)) is inversely proportional to the highest sensed atrial rate. Thus, the highest sensed atrial rate in beats per minute (bpm) is 60,000/TARP in milliseconds. In some DDD pacemakers the MTR equals the highest sensed atrial rate. When the intrinsic atrial rate continues to increase above the MTR, every other P-wave coincides with the TARP. P-waves occurring during the TARP are not sensed and thus are not tracked. Because the pacemaker responds to every other P-wave, the paced ventricular rate plummets to a value which is one half of the intrinsic atrial rate. This is called a fixed block or a 2:1 block upper rate response.
Other pacemakers have an independently programmable MTR interval (MTRI). It is longer than the TARP hence the MTR is below the highest sensed atrial rate. When the intrinsic atrial rate exceeds the MTR but is below the highest sensed atrial rate, each P-wave will be sensed but the timing circuit of the pacemaker will not allow the paced ventricular rate to exceed the programmable MTR. This results in a progressive lengthening of the P-sensed to V-paced interval until a P-wave coincides with the TARP. The P-wave occurring during the TARP is not sensed and hence is not tracked. This results in a pause followed by a repeat of the previous series of PV complexes. This is called pacemaker Wenckebach upper rate behavior.
In both of the above examples atrial events occurring during the TARP are not tracked by the pacemaker for the purpose of pacing the ventricles because pacing the ventricles at an intrinsic atrial rate exceeding the MTR may be dangerous. Instead, as described above, only atrial events occurring outside the TARP are tracked in order to maintain the ventricular pacing rate at a safe level.
Advancements in pacemaker technology have been driven by a desire to approximate true physiological cardiac activity through pacing. In recent years many developments in pacemaker technology, such as rate-responsive pacing, have enabled pacemakers to better emulate some of the functions of the healthy heart. In addition, a number of the developments have improved the comfort of a patient with an implanted pacemaker, especially during the time when the patient's intrinsic atrial rate is high. One upper rate response is known as "fallback behavior." The fallback behavior response differs from other upper rate responses, such as mode-switching, in that the fallback behavior minimizes the patient's discomfort during high intrinsic atrial rate operation by reducing a high ventricular pacing rate to a lower rate in a gradual rather than a sudden manner. The fallback behavior is particularly advantageous for patients for whom sudden drops in the ventricular pacing rate may cause bothersome palpitations or other symptoms.
Another common upper rate response is mode switching. Mode switching is an upper rate response whereby when the intrinsic atrial rate exceeds the MTR the pacemaker automatically switches the pacemaker's mode of operation, for example from a P-wave tracking mode to a non-P-wave tracking mode. Mode switching is described in greater detail in commonly assigned U.S. Pat. No. 5,549,649, issued Aug. 27, 1996, entitled "Programmable Pacemaker Including an Atrial Rate Filter for Deriving a Filtered Atrial Rate Used for Switching Pacing Modes," which is hereby incorporated by reference in its entirety.
As an upper rate response, the fallback behavior is usually invoked when the intrinsic atrial rate exceeds the MTR. Thus, in most previously known pacemakers in which fallback was present, the MTR is also a fallback initiating rate (FIR). Since pacing the ventricles at a rate exceeding the MTR may be dangerous, when the intrinsic atrial rate exceeds the MTR the typical fallback behavior response causes the pacemaker to gradually reduce the ventricular pacing rate to a pre-set programmable fallback rate (FR). The fallback rate is usually a fairly low value, closer to a base rate (BR) than to the MTR. The base rate is typically the minimum rate at which the ventricles and the atria may be paced. The reduction of the ventricular pacing rate to the fallback rate is usually performed by progressively extending the ventricular pacing interval of each cardiac cycle. The pacemaker then paces the ventricles at the fallback rate as long as the intrinsic atrial rate is above the MTR. When the intrinsic atrial rate drops below the MTR the pacemaker sets the ventricular pacing rate equal to the intrinsic atrial rate and resumes 1:1 AV synchronous pacing.
The fallback initiating rate of previously known pacemakers may differ from the MTR but is not typically set above the MTR (even though intrinsic atrial rate exceeding the MTR may be monitored). This arrangement may be problematic for patients whose normal heart rates may slightly exceed the MTR for periods of time. Patients who engage in physical activity, young patients, and patients in an excited emotional state may all exhibit temporary increases in intrinsic atrial rate--where the intrinsic atrial rate exceeds the MTR, but is not indicative of a pathological arrhythmia. As a result, an active patient whose intrinsic atrial rate slightly exceeds the MTR during physical activity or an excited emotional state would be paced at the lower fallback rate which, if insufficient for the activity level, would be inappropriate. Thus, most previously known pacemakers equipped with the fallback behavior response do not distinguish between a high intrinsic atrial rate due to physical activity or an excited emotional state, and a high intrinsic atrial rate due to a pathological arrhythmia. As a result, most previously known pacemakers initiate the fallback behavior response even if the patient is experiencing a temporary high physiologic intrinsic atrial rate which is non-pathologic.
Another drawback of the previously known fallback behavior response is that during fallback operation, the fallback behavior response causes the pacemaker to switch from pacing at the fallback rate to pacing at the intrinsic atrial rate as soon as the intrinsic atrial rate drops below the MTR. This approach is problematic because if the intrinsic atrial rate drops below the MTR for a few cycles and then increases to a rate above the MTR, the patient will experience an increase and then a drop in the ventricular pacing rate over several cycles as the pacemaker attempts to leave and re-enter the fallback behavior. This may occur when a patient is engaging in sporadic physical activity. For example, if a patient is jogging, the patient's intrinsic atrial rate may exceed the MTR while the patient is moving, but may drop below the MTR if the patient pauses to rest. As a result, the patient will experience an increase in the pacing rate when the patient stops moving (since the intrinsic atrial rate temporarily drops below the MTR and the fallback behavior is exited), and a decrease in the pacing rate when the patient once again begins to jog (since the intrinsic atrial rate exceeds the MTR, and the fallback behavior is initiated).
Finally, an apparently low intrinsic atrial rate during a number cardiac cycles may cause the pacemaker to exit the fallback behavior under the assumption that the intrinsic atrial rate dropped below the MTR for more than a few cycles. As a result, the patient may be exposed to a frequent variation of pacing rates as the pacemaker exits and re-enters the fallback behavior with the attendant loss of AV synchrony when fallback behavior is engaged.
Thus it would be desirable to provide a pacemaker capable of initiating the fallback behavior at a programmable rate higher than the MTR, when the patient is likely to be experiencing a pathological arrhythmia. It would also be desirable to provide a pacemaker which would only exit the fallback behavior if programmable fallback exit criteria were met.